Impairment Provisions (Summary of Impairment Provisions Narrative) (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment provision on properties	$ 27,865		$ 9,669		$ 4,688
Provision for impairment	27,865		9,669		1,773
Land held for development or available for sale
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment provision on properties	23,300
Provision for impairment	23,285	[1]	327	[1]	1,387	[1]
Wholly-owned income producing properties
Impaired Long-Lived Assets Held and Used [Line Items]
Provision for impairment	$ 4,600

[1] In 2014, changes to development plans and to estimated fair values triggered an impairment provision of $23.3 million. Refer to Note 1 under Accounting for the Impairment of Long-Lived Assets for a discussion of inputs used in determining the fair value of long-lived assets.